Cell preservation method for pluripotent stem cells

ABSTRACT

The present invention provides a method for freezing a stem cell or a cell derived therefrom, the method including the steps of providing a cell suspension, performing ice nucleation on the cell suspension, and lowering the temperature of the ice nucleated cell suspension to a temperature sufficiently low to allow long term storage of the stem cell. The method is preferably used for the cryopreservation of human embryonic stem cells.

Cross-Reference to Related Applications

This application is a national state entry of PCT/AU05/00783, filed onJun. 2, 2005, published as WO 2005/118785 on Dec. 15, 2005, which is aninternational application of Australian Provisional Application No.2005902558 filed on May 18, 2005 and Australian Provisional ApplicationNo. 2004902933, which are hereby incorporated by reference in theirentirety.

FIELD

The present invention relates to a method of preserving cells byfreezing. More specifically, the method is useful for thecryopreservation of stem cells.

BACKGROUND

Long-term storage of animal cells and tissues is of widespread criticalimportance to the research and biomedical fields. Cryopreservation ofcells and tissues is useful, for the long-term storage of cell lines toprovide an unchanging population of cells; and the storage ofpopulations of cells for research or medical purposes.

It is widely held that animal cells can be stored indefinitely once theyreach liquid nitrogen temperature (−196° C.). It has beenwell-established, however, that the freezing process itself results inimmediate and long-term damage to cells with the greatest damageoccurring to cells as they traverse the intermediate zone of temperature(−15° C. to −60° C.) during cooling and thawing (Mazur, Am. J. Physiol.,247:C125-142, 1984). The primary damaging physical events that can occurto cells during the process of freezing include dehydration andintracellular ice crystal formation. During freezing, solute is rejectedfrom the solid phase producing an abrupt change in concentration in theunfrozen portion of solution. A biological cell responds to thisperturbation by dehydrating to reach a new equilibrium state betweenintracellular and extracellular solutions. At high cooling rates,equilibrium cannot be maintained because the rate at which the chemicalpotential in the extracellular solution is being lowered is much greaterthan the rate at which water can diffuse out of the cell. The end resultof this imbalance is that intracellular ice formation is observed whichis lethal to the cell (Toner, J. of Applied Phys., 67:1582-1593, 1990).At low cooling rates, cells are exposed for long periods of time at highsubzero temperatures to high extracellular concentrations resulting inpotentially damaging high intracellular concentrations (Lovelock,Biochem. Biophys. Acta, 10:414-446, 1953).

There have been attempts in the art to incorporate the process ofvitrification into methods of cryopreserving cells. The aim ofvitrification is to lower the temperature of a cell suspension whileavoiding the formation of ice crystals by the use of viscous orconcentrated liquid solutions. This approach is fundamentally differentto standard methods of freezing that concentrate more so on carefullycontrolling the formation of ice crystals Methods incorporatingvitrification have shown some promise however recoveries can be poor.Furthermore, the methods are not amenable to automation, and thereforequality control can be difficult. Another problem is that compounds suchas polyethylene glycol are required in the vitrification solution. Afurther problem with vitrification is that the vessels used severelylimit the amount of material that can be frozen. Additionally, thecommonly used “open straws” do little to avoid the possibility ofmicrobial cross-contamination of the material to be frozen.

The clinical and commercial application of cryopreservation for certaincell types is limited by the ability to recover a significant number oftotal viable cells that function normally. Significant losses in cellviability are observed in certain primary cell types. Examples offreeze-thaw cellular trauma have been encountered with cryopreservationof hepatocytes (Borel-Rinkes et al., Cell Transplantation, 1:281-292,1992) porcine corneas (Hagenah and Bohnke, 30:396-406, 1993), bonemarrow (Charak et al., Bone Marrow Transplantation, 11:147-154, 1993),porcine aortic valves (Feng et al., Eur. J. Cardiothorac. Surg.,6:251-255, 1992) and human embryonic stem cells (hESCs;http://www.wicell.org/forresearchers, FAQs—Culturing Human ES Cells:FAQs 4 & 8; Reubinoff et al., Human Reprod, 16(10):2187-2194, 2001).

The regulatory requirements for producing clinically acceptable hESCspresent unique characteristics and accompanying challenges. For example,for a hESC to be useful in routine therapeutic applications it will benecessary to generate and store cells in a Master Cell Bank from asingle hESC source. Compliance will ensure quality assurance and safetytowards maximizing clinical efficacy, the primary mandate for the Foodand Drug Administration (FDA). A suitable cryopreservation method thatsatisfies existing and future regulations under Good Tissue Practice(GTP) and Good Manufacturing Practice (GMP) will be essential to themanufacture and use of viable material for cell based therapy. Thus, astandardized procedure with validated components, free from sensitisingreagents such as certain animal sera and selected proteins, performedunder conditions designed to minimize contamination with adventitiousagents, and amenable to high throughput processing for production oflarge cell banks is a necessary prerequisite.

Cryopreservation protocols typically require the use of cryoprotectiveagents (“CPAs”) to achieve improved survival rates for animal cells. Avariety of substances have been used or investigated as potentialadditives to enhance survival of cells in the freezing process. Othersubstances used include sugars, polymers, alcohols and proteins. CPAscan be divided roughly into two different categories; substances thatpermeate the cell membrane and impermeable substances. One mechanism ofprotection results from reduction in the net concentration of ionicsolutes for a subzero temperature when a CPA is present. Thiscolligative effect is true for all substances that act as a CPA (Fahy,Biophys. J. 32:837-850, 1980). The addition of a CPA however, changesthe ionicity of the solution. Both tissues and intact organs can exhibitreduced cellular viability when exposed to sufficiently large stepchanges in external osmolarity produced by introduction of a freezingsolution (Pegg, Cryobiology, 9:411-419, 1972). In addition, long termexposure to even low concentrations of certain CPAs at room temperatureis potentially damaging (Fahy, Cryobiology, 27: 247-268, 1990).

Another media component routinely added to freezing media to reduce celldamage and death during freezing and thawing is serum. This additive,however, is highly complex and may add a number of factors (known andunknown), which may interfere with or alter cell function. Othernon-permeating protective agents such as ethylene glycol, polyvinylpyrrolidone (Klebe and Mancuso, In Vitro, 19:167-170, 1983) sucrose, andculture medium (Shier and Olsen, In Vitro Cell Dev. Biol., 31:336-337,1995), have been studied for their effectiveness as cryoprotectiveagents for cells with variable results. U.S. Pat. No. 4,004,975 toLionetti et al. discloses the cryopreservation of leukocytes fromcentrifuged blood in a solution of hydroxyethyl starch anddimethylsulfoxide. U.S. Pat. No. 5,071,741 to Brockbank and PCT WO92/08347 to Cryolife, published May 29, 1992, disclose the use ofalgae-derived polysaccharides such as agarose and alginate in acryoprotective cell medium. U.S. Pat. No. 5,405,742 to Taylor disclosesa solution for use as a blood substitute and for preserving tissue thatincludes dextran.

PCT WO 95/06068 discloses the use of polysaccharides to improvehematopoietic functions and serve as a radioprotective agent. The use ofgum arabic, cherry resin and apricot resin in ewe semen freezing mediumis disclosed in Platov et al. (Ovtsevodstvo, 10:38-39, 1980, abstract).Holtz et al. (Proc. Fourth Intern. Symp. Repr. Phys. Fish, 1991)discloses the use of saccharides such as glucose and sucrose in thecryopreservation of trout semen. Hill et al. (J. Lab. Clin. Med.,111:73-83, 1988) discloses the use of arabinogalactan to obtain washedmurine platelets by centrifugation. Maisse (Aquat. Living Resour.,7:217-219, 1994) discloses a study of the effect of carbohydrates suchas glucose and maltose on the cryopreservation of trout sperm. Isotonicsucrose in combination with calf serum has been used in a medium for thecryopreservation of animal cells (Shier and Olsen, In Vitro Cell. Dev.Biol., 31:336-337, 1995).

Even in consideration of the many years of research in the field ofcryopreservation, there is still a need in the art for alternative andimproved methods and compositions for freezing cells. There is a specialneed for methods suitable for hESCs. hESCs have the potential to developinto all or nearly all of the more than 200 cell types in the humanbody. They have much therapeutic value for treating disease andregenerating damaged tissues and organs. Their clinical potential,however, hinges on their ability to be easily and reliably passaged,frozen, transported, stored and used.

Like many cells used in biomedical research, embryonic stem cells arecurrently stored and transported in a cryopreserved state in a liquidnitrogen bath. When researchers thaw the cells for use in the lab,however, less than 1% remain viable. The few surviving cells must beplaced in culture and painstakingly tended to for weeks before newcolonies are abundant enough to be useful for experiments or therapy.The low survival rate makes working with the stem cells time and labourintensive. Furthermore because so few cells survive freezing, naturalselection may be altering cell lines in unknown and undesired ways.

In addition, to satisfy existing and future requirements for GTP and GMPand provide quality assurance for the therapeutic utility of stem cells,there is a need for a cryopreservation method with validated components,free from sensitising reagents such as certain animal sera and selectedproteins, performed under conditions designed to minimize contaminationwith adventitious agents, and amenable to high throughput processing forproduction of cell banks.

It is therefore an aspect of the invention to overcome a problem of theprior art to provide improved methods for the cryopreservation of stemcells, ensuring suitable post-freeze/thaw cell viability and cellquality for the therapeutic utility of stem cells.

The discussion of documents, acts, materials, devices, articles and thelike is included in this specification solely for the purpose ofproviding a context for the present invention. It is not suggested orrepresented that any or all of these matters formed part of the priorart base or were common general knowledge in the field relevant to thepresent invention as it existed in Australia before the priority date ofthis application.

SUMMARY OF THE INVENTION

Stem cells are primitive cells known in the art for their ability todifferentiate into mature functional cells of the body. Accordingly,these cells are proposed to have use in therapy of the human body indiseases where the subject's own cells are deficient, damaged or simplynot present. Thus, where a replacement cell is required, a stem cell maybe used as a starting point for generating the mature cell, tissue ororgan required. For this scenario to become a reality in the clinic itis necessary to provide methods for cryopreserving stem cells.

Accordingly, in a first aspect the present invention provides a methodfor freezing a stem cell or a cell derived therefrom, the methodincluding the steps of providing a cell suspension, performing icenucleation on the cell suspension, and decreasing the temperature of theice nucleated cell suspension to a temperature sufficiently low to allowlong term storage of the stem cell. Applicants have found that bydecreasing the temperature of a cell suspension in the manner definedabove, cells of enhanced viability are provided after thawing.

The methods for cryopreservation described herein are particularlyamenable to implementation using a programmable freezer. This automatedapproach typically provides a consistent outcome in terms of cellquality that can be validated for compliance with the code of GoodManufacturing Practice, which is a component of the regulatory packagethat would be required for registration with an authority such as theFDA. Importantly, the Applicant has shown that cryopreservation can beachieved without the use of exogenous biological material as acryoprotectant. Serum is often included in liquid medium used forfreezing cells, however due to the inherent problems in standardisingserum difficulties may arise in registering a therapeutic including thisbiological. The method is particularly advantageous for an embryonicstem cell and a progenitor cell that is partially differentiated andderived from an embryonic stem cell. More particularly the presentapplication demonstrates that human ESCs can be frozen for extendedperiods of time and upon thawing retain important characteristics ofsuch as viability and pluripotency.

Also provided by the present invention are frozen and thawed cellsproduced by the methods of the invention, as well as pharmaceuticalcompositions incorporating the cells, and methods of treatment using thecells.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows post-frozen/thaw hESC recovery at day 8 of culture.Representative photomicrographs of bright field images and Oct-4, TRA1-60 and TRA 1-81 labelling of post frozen/thaw hES-2 cells illustraterecovery following freezing, thaw and reculture. Left and right panelsrepresent replicate cultures.

FIG. 2 shows post-frozen/thaw hESC culture maintained through multiplepassages. Representative photomicrographs of bright field images (leftand right panels represent replicate cultures) and Oct-4, TRA 1-60 andTRA 1-81 labelling of post frozen/thaw hES-2 cells illustrate recoveryfollowing freezing, thaw and reculture.

FIG. 3 shows post-frozen/thaw hESC recovery following short or longerterm cryostorage. Representative photomicrographs of pre and postfrozen/thaw hES-2 cells illustrate cell recovery following freezing,thaw and reculture. Left and right panels represent replicate cultures.

FIG. 4 shows post-frozen/thaw hESC recovery with mechanical cellpassaging. Representative photomicrographs of pre and post frozen/thawhES-2 cells illustrate cell recovery following freezing, thaw andreculture. Panels represent replicate cultures

FIG. 5 shows post-frozen/thaw hESC recovery with mechanical cellpassaging. Representative photomicrographs of post frozen/thaw hES-2cells, illustrate GCTM-2, Oct-4 and TRA 1-81 labelling followingfreezing, thaw and reculture.

FIG. 6 shows post-frozen/thaw recovery of multiple hESC lines maintainedthrough multiple passages. Representative photomicrographs of GCTM-2,Oct-4 and TRA 1-81 labelling of post frozen/thaw hES-2, hES-3, and hES-4cells illustrate cell recovery following freezing, thaw and reculture.

FIG. 7 shows flow cytometric analysis of post-frozen/thaw hESC recoveryfollowing 2 months storage in liquid nitrogen. Representative histogramplots derived from gated events of flow cytometry, illustrates cellnumber (Y-axis) and Oct-4, TRA 1-60, TRA 1-81, SSEA-3, SSEA-4 and SSEA-1labelling (X-axis) of hES-2 cells following freezing, thaw andreculture.

FIG. 8 shows a determination of growth rates of post-frozen/thawedhESCs. Representative growth profiles of post frozen/thaw hESCs,illustrating recovery with normal doubling rates (Study 1: ▪/□, t_(d)=35hr; Study 2: ▴/Δ, t_(d)=31 hr) following freezing, thaw and reculture.Viable cell numbers are represented by filled symbols, and % viabilityof total cells counted by open symbols. Each point represents themean±SEM of three determinations. Each profile represents a replicatestudy.

FIG. 9 shows in vivo differentiation of post-frozen/thaw hESCs.Representative photomicrographs illustrate teratoma sections, derivedfrom xenografting of hES-2 cell clusters following freezing, thaw andreculture.

FIG. 10 shows in vitro differentiation of post-frozen/thaw hESCs.Representative data illustrates post-freeze/thaw differentiation ofhESCs.

FIG. 11 shows post-frozen/thaw recovery of differentiated hESCs.Representative data of post-freeze/thaw illustrates viability ofdifferentiated cells derived by formation of embryoid bodies (EBs) fromhESCs together with a cardiomyocyte differentiation protocol.

FIG. 12 shows efficacy of cryopreservation medium for maintainingpre-frozen hESC viability. Representative profiling of hESC illustratesviability over time (0-3 hours) while stored at 4° C. incryopreservation medium. Data are presented as viable and non-viablecell numbers over time, and are taken from 3 separate studies.

DETAILED DESCRIPTION OF THE INVENTION

In a first aspect the present invention provides a method for freezing astem cell or a cell derived therefrom, the method including the steps ofproviding a cell suspension, performing ice nucleation on the cellsuspension, and decreasing the temperature of the ice nucleated cellsuspension to a temperature sufficiently low to allow long term storageof the stem cell.

The applicant has found that the method is surprisingly effective inpreserving stem cells for extended periods of time. The method may becarried out manually, however it is preferably carried out using afreezer capable of executing a freezing program.

Upon thawing of the cells, it is found that many cell parameters aresubstantially unchanged, such as viability and the ability todifferentiate under appropriate stimulus. This is in contrast to methodsdescribed in the prior art that have deleterious effects (Reubinoff etal., Human Reprod, 16(10):2187-2194, 2001; Richards et al., Stem Cells.2004; 22(5):779-89).

The starting material for cryopreservation is generally a stem cellculture taken from a laboratory incubator. In terms of preparation forfreezing, the cells may be disaggregated to form a suspension by eitherenzymatic or mechanical means (see FIG. 4). Similarly, passaging afterthawing can be achieved mechanically (see FIG. 5).

The skilled person will understand that there are many standard methodsfor preparing a cell suspension for the purposes of freezing or furtherpassaging the cells. However in a preferred form of the method the cellsare prepared by a method including the steps of exposing a cell cultureor aggregate to an amount of collagenase for a length of time sufficientto produce cell clumps, exposing the cell clumps to an excess volume ofhESC media, disrupting the cell clumps mechanically to generate uniformsize cell clumps, pelleting the cell aggregates and resuspending thecells in a solution containing a cryoprotectant. In a preferred form ofthe method the solution is substantially devoid of extraneous biologicalmaterial such as serum. Indeed, the ability to successfully cryopreservecells is an advantage of the present invention. It is very difficult toregister a biological for clinical use if it includes or has been incontact with an undefined substance such as animal serum. Substancessuch as serum may also contain pathogens such as bacteria and viruses.In a preferred form of the method the solution is the same as or similarto CryoStor™ CS5 or CryoStor™ CS10 solutions. CryoStor™ CS5 solutioncontains a final concentration of 5% DMSO as a cryoprotectant, which canbe further augmented by the addition of further DMSO to achieve a finalconcentration of 10%. Alternatively, CryoStor™ CS10 can be employed,which already comprises a final concentration of 10% DMSO. The presentlydescribed preferred freezing solutions are FDA approved for utilizationin cell therapy and tissue engineering arena.

As a precursor step to ice nucleation, the cell suspension may besubjected to a “cold activation step”. This typically involves coolingthe suspension to a temperature from about 4° C. to about −12° C.Preferably the suspension is cooled to about −8° C. at a rate of about−1° C./min for the cold activation step. A “soak” step may then followwhereby the cell suspension is maintained at the temperature used forcold activation (typically −8° C.) for a period of from about 5 min toabout 10 min.

Following the “cold activation step” to achieve thermal equilibrium, icenucleation is initiated. Ice nucleation occurs by decreasing thetemperature of a cell sample to a nucleating point. Following icenucleation, the temperature of the cell sample is lowered from thenucleating point to the solidification point.

Where a programmable freezer is used to implement the method, the icenucleation step is performed by providing appropriate instructions tothe freezer. Preferably the ice nucleation step is performed by loweringthe temperature of the cell suspension from the temperature at theconclusion of the cold activation step to a temperature of about −10° C.to about −12° C. More preferably the temperature is lowered to about−10.9° C. The temperature is typically lowered rapidly at a rate of fromabout −15° C./min to about −55° C./min. More preferably the rate isabout −35° C./min to about −38° C./min.

In one form of the invention the ice nucleation step includes loweringthe temperature of the cell suspension to a temperature of from about−11° C. to about −13° C. Preferably the temperature is lowered to atemperature of about −12.1° C. Typically, the rate at which thetemperature is lowered is from about −5° C./min to about −15° C./min.Preferably the rate is −9° C./min. At the end of the ice nucleation stepthe temperature of the cell suspension is typically about −12.1° C.

In another embodiment of the method the ice nucleation step is performedmanually. Where a programmable freezer is used, the manual icenucleation step may be performed by halting the programmed sequence ofthe freezer at an appropriate point, removing the straw containing thecell suspension, touching the straw with liquid nitrogen-cooled forceps,returning the straw to the freezer, and instructing the freezer toresume the freezing program. The skilled person will be able to identifyother means and contrivances to achieve ice nucleation manually, allbeing included in the scope of the present application.

One possible freezer program to accommodate manual ice nucleation isshown in Example 2. Alternatively, ice nucleation may be performedautomatically by the freezer program. This may be done by programmingfurther steps into the freezer program such as those described in steps4 to 8 of the program described in Example 3.

It will be understood that the present method may include a period wherethe cell suspension is left to incubate for a period of time to allow agiven process to at least partially complete. For example, following theice nucleation step the cell suspension may be kept at the temperatureat which ice-nucleating is performed to allow the formation of anadequate seed or seeds in the cell. In a preferred form of the method, aperiod of about 5 min is allowed for adequate seed or seeds to formbefore proceeding with further cooling.

After the ice nucleation step the method may include a dehydration stepwhereby the temperature of the cell suspension is lowered to atemperature of from about −35° C. to about −38° C. This decrease intemperature is often achieved relatively slowly, and is typicallyimplemented at about −0.8° C./min.

After the ice nucleation or dehydration step, the suspension istypically further decreased rapidly to a temperature of about −180° C.for long term isolation and storage in liquid nitrogen.

Without wishing to be limited by theory, it is proposed that where therate of cooling is too low, cell death arises through extended periodsof exposure to hypertonic conditions. Accordingly, by increasing therate of cooling, the exposure time to hypertonic conditions is decreasedalong with a concomitant reduction in cell damage. Where the rate ofcooling is too fast, cell death arises through intracellular iceformation. Therefore, a preferred rate of cooling may be considered asthe most rapid rate of cooling without intracellular ice formation.

The skilled person will understand that the method may include othersteps including further temperature manipulation or the addition ofreagents.

The present invention has particular applicability to thecryopreservation of mammalian stem cells. Stem cells have two importantcharacteristics that distinguish them from other types of cells. First,they are unspecialized cells that renew themselves for long periodsthrough cell division. The second is that under certain physiologic orexperimental conditions, they can be induced to become cells withspecial functions such as a heart muscle cell or an insulin-producingcell of the pancreas.

In a more preferred form of the invention the stem cell is an embryonicstem cell, which are cultured from cells obtained from the inner cellmass of an embryo. Embryonic stem cells and are pluripotent, having thepotential to develop into nearly all of the tissues in the body. Thisclass of cell appears to have no equivalent cell type in vivo, and isproposed to be a tissue culture artifact (reviewed by Zwaka andThompson, 2005, Development 132(2), 227-233). Embryonic stem cells areknown to exhibit a number of properties that are simply not seen in theintact embryo. For example, although embryonic stem cells retainproperties of early embryonic cells in vitro, no pluripotent celldemonstrates long term self-renewal in vivo. Embryonic cells, oncebrought into tissue culture, are exposed to numerous extrinsic signalsto which they would never be exposed in vivo. Other workers have shownthat ES cells adapt to tissue culture conditions and acquire novelfunctions that allow them to proliferate in an undifferentiated stateindefinitely (Buehr and Smith, 2003, Philos Trans R Soc Lond B Biol Sci358, 1397-1402; Rossant, 2001, Stem Cells 19, 477-482; Smith, 2001, AnnuRev Cell Dev Biol 17, 436-462). Still more preferably the stem cells arehESCs. As discussed elsewhere herein, there is great potential for thetherapeutic use of embryonic stem cells for the treatment of humandisease.

Applicants have found that by implementing the cryopreservation methodsdescribed herein, cells of improved quality are provided for freezingand after thawing when compared with methods of the prior art. In oneform of the method, viability of cells prepared for freezing may bemeasured by monitoring the number of cells still living as a function oftime after exposure to a temperature of 4° C. By this measure, greaterthan about 90% of the cells that were living immediately after preparingthem for freezing prior to freezing were still living after 1 hour at 4°C. (see FIG. 12). Another preferred form of the method provides thatabout 75% of the cells that were living immediately after thawing werestill living after 3 hours at 4° C. (see FIG. 12).

Another advantage of the invention is that upon thawing the cells retainpluripotency, retaining an undifferentiated phenotype. As will be notedin FIG. 1, after thawing hESCs, the cells maintained characteristics ofundifferentiated cells. The cells exhibited non-cystic growth andexpressed Oct-4, TRA 1-60 and TRA 1-81. FIG. 6 confirms this finding byflow cytometry, and demonstrates retention of other markers ofpluripotency such as SSEA-3 and SSEA-4, with negligible retention of thedifferentiation marker SSEA-1.

Of further significance, Applicants have shown that when passaged underappropriate conditions the thawed cells were able to maintainpluripotency. FIG. 2 shows that after five passages, the cells retainedmarkers Oct-4, TRA 1-60 and TRA 1-81. FIG. 7 illustrates a flowcytometric analysis of hESCs whereby the cells maintained markers suchas Oct-4, TRA 1-81, TRA 1-60, SSEA-3, SSEA-4, these markers beingindicative of the pluripotency of a cell, while expressing negligibleSSEA-1, a marker of differentiation.

Cells prepared and cryopreserved according to the methods describedherein have the ability to differentiate in vivo into an endodermal,mesodermal or ectodermal cell. In a more preferred form of the inventionthe cells can differentiate into a cell type selected from the groupconsisting of an hepatic cell, a renal cell, a dermal cell, acardiovascular cell, a neural cell, a skeletal cell, a pancreatic celland a reproductive cell. Most preferably the cells can differentiateinto a cell type selected from the group including a gut epithelialcell, a chondrocyte, an osteocyte and a hair follicle cell (see FIG. 9)The ability to differentiate in vitro has also been shown in FIG. 10where hES-3 cells were differentiated into beta-like cells. The skilledperson will have sufficient knowledge to direct a stem cell down acertain lineage as desired by exposing the cell to one or more cultureconditions or agents.

A further advantage of the invention is that cells do not appreciablydegrade according to the length of storage. FIG. 3 shows that cellsstored for 1 day or 6 days are of the same quality when thawed. Inaddition, FIG. 7 shows cells stored for 2 months remain of high qualityand pluripotent.

While the cryopreservation of cell types hES-2, hES-3 and hES-4 has beendemonstrated herein, the method is proposed to be applicable to thebroader class of stem cells. including but not limited to embryonic stemcells and more particularly human embryonic stem cells. FIG. 6 showsthat cell quality after freezing and thawing is equivalent for hES-2,hES-3 and hES-4 cells. Cytogenetic analysis of the three cell typesrevealed normal karyotypes. The Table below shows data on cytogeneticanalysis of post-frozen/thaw hESCs—Normal karyotype of post frozen/thawhES-2, hES-3 and hES-4, illustrating cell recovery following freezing,thaw and reculture.

hESC Line Replicate Study Result hES-2 1 46XX 2 46XX hES-3 1 46 XX 246XX hES-4 1 46 XY 2 46 XY

The cells also exhibit normal growth rates after cryopreservation andthawing, as demonstrated by FIG. 8, which shows normal doubling ratesfor thawed cells.

The methods of the present invention are also applicable to cellsderived from ES cells, such as progenitor cells, hepatic cells, renalcells, dermal cells, cardiovascular cells, neural cells, skeletal cells,pancreatic cells and reproductive cells. It is within the ability andknowledge of a person of ordinary skill in the art to produce a moredifferentiated cell for cryopreservation from a stem cell. FIG. 11 showsthat progenitor cells derived from hESCs are able to be cryopreserved,thawed and then differentiated into cardiomyocyte-like cells. This is asignificant contribution to the art since for the first time it ispossible to harvest a stem cell from a subject, cryopreserve the cell,and thaw the cell at some time in the future for use in that subject oranother subject without a substantial loss of cell quality. Beforefiling of this application, the art was dependent on methods thatresulted in substantial loss of cell viability leading to a significantloss of cells. While a small number of cells survive thecryopreservation methods of the prior art, it was necessary to multiplypassage the surviving cells to generate a number of cells useful fortherapy. As the skilled person understands, multiple passaging isundesirable given the increased opportunity for the introduction ofmutations in the cell genome. Furthermore, the inclusion of abiologically-derived material as a cryoprotectant necessarily increasesthe danger of exposing the subject to an adventitious pathogen

As the skilled person will understand, the ability to more effectivelycryopreserve stem cells is key to the further development of stem cellsas therapeutics in the treatment of disease. In order to properlyutilise a stem cell as a therapeutic in routine medical practice it willbe necessary to provide a large bank of stem cells, the cells in thebank being derived from a single cell population. It is only by thisapproach that the physician can be assured of the origin, safety andefficacy of any stem cell used in therapy.

The method may be conducted in any apparatus capable of achieving thetemperatures and rates of cooling required. Generally speaking, apurpose-built programmable biological rate freezer is the most suitablecontrivance. An exemplary freezer is the MC-012 Planer BiologicalFreezer (Cat No Kryo 10-16 or Kryo 360-1.7). The use of computercontrolled devices provides a high degree of reproducibility andtherefore minimal batch-to-batch variation.

The cell suspension may be provided in any suitable receptacle for thepurposes of the instant invention. The size of the receptacle may besuch so as to facilitate rapid freezing of the sample contained therein,using methods of the present invention. Furthermore, the receptacle islikely be made of a material that will permit cold or heat to be rapidlyconducted from an outer surface of the receptacle to the samplecontained within the receptacle. In particular embodiments of thepresent invention, the material may also permit storage of the frozensample at temperatures less than −130° C. including, but not limited to,a temperature in the order of −190° C. to −200° C. An exemplary materialthat may be used under these conditions is polypropylene. Polypropylenehas been used in known cryopreservation receptacles, for examplecryotubes. In further embodiments of the present invention, thethickness of the walls of the receptacle will be sufficiently thin topermit rapid freezing of the sample contained therein, using methods ofthe present invention. A receptacle that may be used in particularembodiments of the present invention may include, but not be limited to,a straw such as a straw used in cell or embryo freezing. The receptaclemay also be closable by any means, including but not limited to heatsealing of its ends. An advantage of the present invention is that it ispossible to conduct the freezing process with or without the receptableclosed since it is not necessary to monitor a biophysical parameter ofthe cell suspension during freezing. For the purposes of maintainingsterility of the cell suspension, it is generally preferred to conductthe method with the receptacle closed.

The skilled artisan will be familiar with many such receptacles capableof withstanding the very low temperatures required for thecryopreservation of cells, with an example being MC-009 CBS HighSecurity Straw Sterile (Cryo Bio System Cat No 014651). The straws maybe closed by the manual sealing unit MC-010 SYMS (Cryo Bio Systems CatNo 007213) for CBS straws. The presently described exemplary straws areFDA approved for utilization in cell therapy and tissue engineeringarena.

In another aspect the present invention provides a frozen cell, whereinthe cell has been frozen according to a method as described herein.

A further aspect of the present invention provides a thawed cell,wherein the thawed cell has previously been frozen according to a methodas described herein. Thawing of the cell can be accomplished by anysuitable method known to the skilled artisan. In a preferred embodimentthe cells are rapidly brought to 37° C. by submerging the receptaclecontaining the cell suspension in a water bath. Once the suspension isthawed, the cells may be pelleted and washed in any suitable medium. Onmany occasions it will be advantageous to resuspend the cells in themedia required for further culturing of the cells.

In another aspect the present invention provides a method of treating asubject in need of a stem cell transplant, the method includingadministering to the subject an effective amount of a frozen or thawedcell as described herein. There are a number of diseases that may betreated by the administration of stem cells with promising areasincluding neurodegenerative disorders, heart disease and diabetes.

In another aspect the present invention provides a method of growing anorgan or tissue in vitro, the method including use of a frozen or thawedstem cell as described herein. Stem cells may be used to manufacturecomplete organs, parts of organs, or tissues for subsequent implantationinto a subject. For example stem cells may be used to generate skingrafts for use on burns victims. Stem cells may also be used to generateentire organs such as the pancreas. This would have use in the treatmentof insulin dependant diabetes mellitus.

Given the therapeutic utility of ES cells, it will be clear that thecells of the present invention will have use in the form of apharmaceutical composition. Accordingly, the present invention providesa pharmaceutical composition including a frozen or thawed cell asdescribed herein in combination with a pharmaceutically acceptablecarrier. The skilled person will be familiar with many substances thatmay be combined with the cells of the present invention to enhancestability and viability of the cells in the composition. As an example,substances such as buffers and salts may be added to adjust the pH andtonicity of the composition as a whole.

In another aspect the present invention provides the use of a frozen orthawed cell as described herein in the preparation of a medicament forthe treatment or prevention of a condition requiring a stem celltransplant.

Examples of the methods used in the present invention will now be morefully described. It should be understood, however, that the followingdescription is illustrative only and should not be taken in any way as arestriction on the generality of the invention described above.

EXAMPLES Example 1: Preparation and Cryopreservation of hESCs

The methods were performed in a laboratory having standard cell biologyequipment (biosafety hoods, pipettes, aspiration pumps, 5% CO₂ incubatorat 37° C., 70% ethanol spray, centrifuge, tubes, dissecting microscopewith warm stage, liquid nitrogen storage vessel etc). Before commencingthe methods, the following reagents were prepared: live hESC colonies,hESC culture media (hESC media), Phosphate Buffered Saline (PBS; Gibcocatalogue number 14040-133), Collagenase IV (20 mg/ml stock preparedin-house; Serva catalogue number 17458.03), Dimethylsulfoxide (DMSO;Sigma-Aldrich product number D1435/D2650), and CRYOSTOR CS5™Cryopreservation Solution (BioLife Solutions catalogue number99-610-DV). The hESC media consisted of 20% KnockOut Serum Replacementcomprising bovine serum albumin (Invitrogen/Gibco, catalogue number04-0095) or human serum albumin, 78% KnockOut DMEM (Invitrogen/Gibco,catalogue number 10829-018), 1% L-Glutamine (Invitrogen/Gibco, cataloguenumber 25030-081), 1% NEAA (Invitrogen/Gibco, catalogue number11140-050), and 50 ng/mL bFGF (Strathmann Biotech AG, catalogue number9511060).

The following specialised hardware was provided in the laboratory: CBSHigh Security Straw Sterile (Cryo Bio System catalogue number 014651),SYMS manual sealing unit for CBS straws (Cryo Bio Systems cataloguenumber 007213), a Planer Biological Freezer (catalogue number Kryo 10-16or Kryo 360-1.7).

Cells were harvested using Collagenase IV. Briefly, media was aspiratedfrom cell culture vessels containing hESCs and rinsed with PBS+.Collagenase IV working solution was added to cell culture vessels andincubated at 37° C., 5% CO₂ for 8-10 min. hESCs were gently scraped fromthe culture surface and hESC media was added and the cell suspension wasgently agitated.

Digested cell clumps (variable size) were transferred to a tube withhESC media and disaggregated further to generate more uniform cellclumps. Cells were then pelleted by centrifugation.

Media was aspirated and each cell pellet resuspended in freezingsolution. Cells were maintained in freezing solution on ice for 10 min.

Cells in freezing solution were transferred to CBS™ High Security Strawsusing a P200. Each straw was held at the top, where the safety stopperensured that sterility was maintained throughout the filling procedure.The open end of a straw was protected from any contamination. The endsof a loaded straw were carefully sealed using the sealing unit. Strawswere labelled and maintained on ice until all “loaded” straws weresealed.

Once the freezing chamber of the control rate freezer had reached thestart temperature (+4° C.), straws were loaded into the unit. “Run” wasselected from the main menu.

For manual ice nucleation at −8° C., after soaking the cells at −8° C.for 5 min: the cells were ice nucleated by application of liquidnitrogen cooled forceps to the outside of the straws. “Enter” waspressed on the control rate freezer control panel to resume the profile.Where “Automated” ice nucleation of the cells was implemented a rapidcooling ramp program was used (for example program see the table inExample 3).

Straws were then removed from the control rate freezer and plunged toliquid nitrogen, where they were stored in a liquid nitrogen vesselcanister.

Example 2: Freezer Program Incorporating Manual Ice Nucleation

The following program was used with a Planer Biological Rate Freezer andincluded a manual ice nucleation step.

RATE STEP FROM TO (° C./ TIME (RAMP) (° C.) (° C.) MIN) (MIN)DESCRIPTION 1. +4 +4 0 5 HOLD 2. +4 −8 −1.0 Cold activation 3. −8 −8 0 5Soak 4. −8 −8 0 Manual ice nucelation 5. −8 −8 0 5 Hold after icenucleation 6. −8 −38 −0.8 Dehydration 7. −38 −100 −10.0 Pre-plunge rapidcool 8. −100 −180 −35.0 Plunge

Example 3: Freezer Program Incorporating Automated Ice Nucleation

The following freezing program was used with a Planer Biological RateFreezer, and including a automatic ice nucleation step.

Example 4: Thawing hESCs

A hESC loaded straw was removed from liquid nitrogen to a portablereceptacle containing liquid nitrogen. The straw was thawed immediatelyby gently swirling in a 37° C. water bath until only a small ice pelletremained (10-20 sec).

The straw was completely submerged in 70% ethanol. Once dry, the end ofthe straw was cut off about 0.5 cm from the hydrophobic plug, using asterile scalpel or pair of scissors. The other end of the straw was cutoff above the seal.

The contents of the straw were emptied into a conical centrifuge tubewith 37° C. hESC media. The straw was then rinsed with an additionalvolume of hESC media. The cells were centrifuged at 1000 rpm for 3 min,the supernatant was aspirated, and the cell pellet resuspended in 37° C.hESC media. The cell suspension was transferred to prepared cell culturevessels containing 37° C. hESC media for subsequent culture

Example 5: Effect of Cryopreservation on Cellular Parameters

In order to show the general quality of frozen and thawed cells, hESCswere cultured on human fibroblast feeder cells in hESC media,enzymatically disaggregated with Collagenase IV, contained inhermetically sealed straws and frozen. After storage in liquid nitrogen,cell samples were thawed and re-cultured for 8 days. FIG. 1 demonstratesthat recovered hESCs were healthy, robust and undifferentiated(non-cystic and immunopositive) cell growth, supporting the efficacy ofthe cryopreservation protocol employed.

To be useful in the laboratory and clinic, it was necessary to show thatthe cryopreservation method did not affect the ability of cells to bemaintained through multiple passages. Briefly, hESC colonies werecultured on human feeders in hESC media, enzymatically disaggregatedwith Collagenase IV, contained in hermetically sealed straws and frozen.After storage for 6 days in liquid nitrogen, cell samples were thawedand re-cultured through 5 passages. It can be noted from the micrographsin FIG. 2 that hESC colonies comprised healthy, robust andundifferentiated (non-cystic and immunopositive) cell growth, supportingthe efficacy of the cryopreservation protocol employed.

To demonstrate that the time of cryostorage did not adversely affectcells when frozen using the cryopreservation method, hESCs were culturedon human fibroblast feeder cells in hESC media, enzymaticallydisaggregated with Collagenase IV, contained in hermetically sealedstraws, frozen and stored for short (1 day) and longer (6 days) term.After storage for 1 day or 6 days in liquid nitrogen, cell samples werethawed and re-cultured for 11 days. FIG. 3 shows that hESC recoverycomprised healthy, robust and undifferentiated/non-cystic cell growth,supporting the efficacy of the cryopreservation protocol employed. In afurther example, FIG. 7 shows cells stored for 2 months or longer remainof high quality and pluripotent, confirming the efficacy of extendedterm storage using the present cryopreservation method

The effect of mechanical cell passage in combination with the inventivemethods was also assessed. hESCs were cultured on human fibroblastfeeder cells in hESC media, mechanically disaggregated, contained inhermetically sealed straws and frozen. After storage in liquid nitrogen,cell samples were thawed and re-cultured for 15 days. FIG. 4 showsrecovered hESCs were healthy, robust and undifferentiated/non-cystic,supporting the efficacy of the cryopreservation protocol employed.

FIG. 5 provides further support for the utility of mechanical passaging.hESCs were cultured on human fibroblast feeder cells in hESC media,enzymatically disaggregated with Collagenase IV, contained inhermetically sealed straws and frozen. After storage in liquid nitrogen,cell samples were thawed and re-cultured through 1 passage usingmechanical transfer. Clearly, hESC recovery comprised healthy, robustand undifferentiated (immunopositive) growth, supporting the efficacy ofthe cryopreservation protocol employed.

The effect of multiple passaging in combination with mechanicaldisaggregation was assessed. hESCs were cultured on human fibroblastfeeder cells in hESC media, mechanically disaggregated, contained inhermetically sealed straws and frozen. After storage in liquid nitrogen,cell samples were thawed and re-cultured through 5 passages again usingmechanical transfer. Immunohistochemistry was performed on day 7cultures following passage 5. Clearly, hESC recovery comprised healthy,robust and undifferentiated (immunopositive) cell growth, supporting theefficacy of the cryopreservation protocol employed (see FIG. 6).

FIG. 7 further shows that the cryopreservation method does not affectthe expression of differentiation-related cellular markers. hESCs werecultured on human fibroblast feeder cells in hESC media, mechanicallydisaggregated, contained in hermetically sealed straws and frozen. Afterstorage in liquid nitrogen, cell samples were thawed and re-cultured.Relative to negative control (i.e. Isotype antibody labelling; left handgreen open plots), most cells sustained high level Oct-4 (˜90%; pinkfilled), TRA1-60 (˜90%; pink filled), TRA 1-81 (˜90; blue open), SSEA-3(60%; pink filled), SSEA-4 (˜70%; pink filled), and negligible SSEA-1(˜5%; pink filled) expression, indicating undifferentiated hESCproliferation. Clearly, data support hESC recovery comprisingundifferentiated (immunopositive) cell growth, supporting the efficacyof the cryopreservation protocol employed.

The effect of freezing and thawing on growth rates of hESCs wasinvestigated. hES-2 cells were cultured on human fibroblast feeder cellsin hESC media, enzymatically disaggregated with Collagenase IV,contained in hermetically sealed straws and frozen. After storage inliquid nitrogen, cell samples were thawed and re-cultured through >9passages. Viable cell number was identified during the exponentialgrowth phase of the final week of culture. Cells were harvested dailyusing Collagenase IV and viable cell numbers were determined by Trypanblue exclusion for calculation of doubling time and plotting as graphsof number verses time. For all studies, cultures comprised high cellviability and reached confluency within the time period studied. Thedata presented in FIG. 8 confirms hESC recovery comprising normalgrowth, supporting the efficacy of the cryopreservation protocolemployed.

The effect of freezing and thawing on cell karyotype was investigated.Briefly, hESCs were cultured on human fibroblast feeder cells in hESCmedia, mechanically disaggregated, contained in hermetically sealedstraws and frozen. After storage in liquid nitrogen, cell samples werethawed, re-cultured through a minimum of 5 passages and harvested.Normal cell karyotype was determined by evaluating Giemsa stainedmetaphase spreads, supporting the efficacy of the cryopreservationprotocol employed. Replicate studies represent separate cell culturesperformed in parallel.

FIG. 9 presents further evidence that hESCs preserved by the presentmethod maintain pluripotency and will differentiate in vivo, and willtherefore be useful in the clinic. hESCs were cultured on humanfibroblast feeder cells in hESC media, mechanically disaggregated,contained in hermetically sealed straws and frozen. After storage inliquid nitrogen, cell samples were thawed, re-cultured, harvested andgrafted into the testes of severe combined immunodeficiency mice togenerate teratomas. Histological analysis indicated that tumorscomprised differentiated tissues of all three embryonic germ layers suchas gut epithelium (endoderm), cartilage and bone (mesoderm), and hairfollicles (ectoderm); supporting hESC pluripotency and the efficacy ofthe cryopreservation protocol employed.

In vitro differentiation of thawed hESCs was studied. hES-3 cells werethawed and expanded through 4 passages. The cells were then placedthrough a 36-day optimized beta-cell differentiation protocol andscreened for the intermediate marker gene pdx-1 on day 20 (left panel),and for C-peptide release into the media on day 36 (right panel). Takentogether, the data shown in FIG. 10 support the efficacy of thecryopreservation protocol employed for preservation of hESCs, able to betransformed after thawing to differentiated cell types such as beta-likecells.

In order to show that the method has utility for cells derived fromhESCs, hES-3 derived embryoid bodies were enzymatically digested anddisaggregated progenitor cells were split into two groups of samples,contained in separate batches of freezing media in hermetically sealedstraws and frozen. After storage for 8 days in liquid nitrogen, cellsamples were thawed, counted, plated and cultured to activecardiomyocytes by directed differentiation. Importantly, afterfreeze/thaw, the majority of cells attached, with beating cardiomyocytesapparent following 5 days of culture. Taken together, data shown in FIG.11 support the efficacy of the cryopreservation protocol employed forpreservation of hESC derived progenitor cells, able to be transformed todifferentiated cells such as cardiomyocyte-like cells.

Efficacy of preparing cells for freezing is clearly an important featureof the method. This was evaluated by culturing hES-2 cells on mousefibroblast feeder cells in hESC media, enzymatically disaggregating themwith Collagenase IV, and suspending them in cryopreservation medium at4° C. over 3 hours. Samples were taken from stock every 20 min,trypsinized and counted using trypan blue exclusion method. Datapresented in FIG. 12 indicate a gradual but acceptable slow decrease inviable cells and corresponding increase in non-viable cells over thetime period measured.

It will be apparent to the skilled person that many routines variationsmay be applied to the various methods and compositions described herein.It is intended that these variations are included with the scope of thepresent application.

The invention claimed is:
 1. A method for freezing a pluripotent stemcell, the method including the steps of providing a pluripotent stemcell suspension, performing ice nucleation on the pluripotent stem cellsuspension by rapidly lowering the temperature of the suspension to anucleating point at a rate from about −5° C./minute to about −55°C./minute to provide an ice nucleated pluripotent stem cell suspension,and lowering the temperature of the ice nucleated pluripotent stem cellsuspension to a temperature sufficiently low to allow long term storageof the pluripotent stem cell, wherein upon thawing, the pluripotent stemcell retains pluripotency, and wherein the pluripotent stem cell is ahuman embryonic stem cell.
 2. The method according to claim 1 whereinupon thawing the pluripotent stem cell is capable of differentiatinginto a cell type selected from the group consisting of a hepatic cell, arenal cell, a dermal cell, a cardiovascular cell, a neural cell, askeletal cell, a pancreatic cell and a reproductive cell.
 3. The methodaccording to claim 1 wherein upon thawing the pluripotent stem cell iscapable of differentiating into a cell type selected from the groupconsisting of a gut epithelial cell, a chondrocyte, an osteocyte, acardiomyocyte-like cell, a beta-like cell, and a hair follicle cell. 4.The method according to claim 1 wherein the ice nucleation step includeslowering the temperature of the pluripotent stem cell suspension to atemperature of from about −11° C. to about −13° C.
 5. The methodaccording to claim 4 wherein the temperature of the pluripotent stemcell suspension is lowered to a temperature of about −12.1° C.
 6. Themethod according to claim 4 wherein the temperature of the pluripotentstem cell suspension is lowered at a rate of from about −5° C./minute toabout −15° C./minute.
 7. The method according to claim 6 wherein thepluripotent stem cell suspension is lowered at a rate of about −9°C./minute.
 8. The method according to claim 4 wherein the temperature ofthe pluripotent stem cell suspension at the end of the ice nucleationstep is about −12.1° C.
 9. The method according to claim 1 wherein thepluripotent stem cell suspension is kept for a period at the temperatureat which ice-nucleation is performed to allow the formation of anadequate seed or seeds in the pluripotent stem cell.
 10. The methodaccording to claim 9 wherein the period is about 5 minutes.
 11. Themethod according to claim 1 wherein the pluripotent stem cell suspensionis subjected to a cold activation step before the ice nucleation step.12. The method according to claim 11 wherein the cold activation stepincludes cooling the pluripotent stem cell suspension to a temperatureof from about −4° C. to about −12° C.
 13. The method according to claim11 wherein the cold activation step includes cooling the pluripotentstem cell suspension to about −8° C.
 14. The method according to claim11 wherein the cold activation step includes cooling the pluripotentstem cell suspension at a rate of about −1° C./minute.
 15. The methodaccording to claim 11 wherein the ice nucleation step is performed bylowering the temperature of the pluripotent stem cell suspension fromthe temperature at the conclusion of the cold activation step to atemperature of about −10° C. to about −12° C.
 16. The method accordingto claim 15 wherein the temperature is about −10.9° C.
 17. The methodaccording to claim 15 wherein the temperature is lowered rapidly at arate of from about −15° C./minute to about −55° C./minute.
 18. Themethod according to claim 17 wherein the temperature is lowered at arate of from about −35° C./minute to about −38° C./minute.
 19. Themethod according to claim 1 wherein the pluripotent stem cell suspensionis subjected to a soak step after the cold activation step and/or beforethe ice nucleation step.
 20. The method according to claim 19 whereinthe soak step includes maintaining the pluripotent stem cell suspensionat the final temperature achieved by the cold activation step for aperiod of from about 5 minutes to about 10 minutes.
 21. The methodaccording to claim 1 wherein following the ice nucleation step, thetemperature of the pluripotent stem cell suspension is lowered from thenucleating point to a solidification point.
 22. The method according toclaim 1 including a dehydration step after the ice nucleation stepwhereby the temperature of the pluripotent stem cell suspension islowered to a temperature of from about −35° C. to about −38° C.
 23. Themethod according to claim 22 the temperature of the pluripotent stemcell suspension is lowered at a rate of about −0.8° C./minute.
 24. Themethod according to claim 1 wherein after the ice nucleation ordehydration step, the temperature of the pluripotent stem cellsuspension is decreased rapidly to a temperature of about −180° C. 25.The method according to claim 1 wherein the pluripotent stem cellsuspension does not contain an exogenous biological cryoprotectant. 26.The method according to claim 25 wherein the exogenous biologicalcryoprotectant is serum.
 27. The method according to claim 1 wherein apluripotent stem cell parameter is substantially unchanged afterthawing, the pluripotent stem cell parameter including viability and/orthe ability to differentiate under appropriate stimulus.
 28. The methodaccording to claim 1 wherein upon thawing the population of pluripotentstem cells the pluripotent stem cell suspension has a viability of up to90%.
 29. The method according to claim 1 wherein upon thawing thepluripotent stem cell retains an undifferentiated phenotype.
 30. Themethod according to claim 1 wherein upon thawing the pluripotent stemcell exhibits non-cystic growth.
 31. The method according to claim 1wherein after thawing the pluripotent stem cell retains a markerselected from the group consisting of Oct-4, TRA 1-60, TRA 1-81, SSEA-3and SSEA-4.
 32. The method according to claim 1 wherein upon thawing thepluripotent stem cell exhibits negligible retention of the markerSSEA-1.
 33. The method according to claim 1 wherein upon thawing thepluripotent stem cell is capable of differentiating in vivo or in vitrointo an endodermal, mesodermal or ectodermal cell.
 34. The methodaccording to claim 1 wherein a pluripotent stem cell stored frozen forat least 2 months retains pluripotency.
 35. The method according toclaim 1 wherein upon thawing the pluripotent stem cell exhibits a normalkaryotype.
 36. The method according to claim 1 wherein upon thawing thepluripotent stem cell exhibits a normal growth rate.